The goal of the proposed research is to advance our knowledge about the psychological processes involved in scientific reasoning, and the development of those processes. The empirical work will be an extension of earlier studies of how both adults and pre-adolescent children discover the rules governing a hidden mechanism on a complex device. It will be guided by conceptualization of scientific discovery that shows how search in two problem spaces (an hypothesis space and an experiment space) shapes hypothesis generation, experimental design and the evaluation of hypotheses. The proposed research will take place on five interrelated fronts, including: Further empirical investigations of how people discover hidden mechanisms in devices: This series of studies will address the role of prior knowledge in the discovery of how a device works, how hypotheses are translated into experiments, how experimental results are translated into theories and hypotheses, the role of memory in scientific reasoning, the effect of different types of instruction on scientific reasoning, and the different strategies that can be used in scientific reasoning. Extension of context from discovery of mechanisms to discovery of properties: Previous research focused on how people discover mechanisms while reasoning about devices. The second set of proposed studies will be set in a radically different domain-reasoning about the chemistry of an unknown substance. The focus here will be on how people discover properties rather than processes, and the extension to a new domain will test the generality of the model. Further investigations of the development of scientific reasoning skills: Studies in both contexts will continue to address developmental issues: What are the underlying processes for inducing general hypotheses from data that appear to be available to adults, but not children? How does the ability to evaluate evidence develop? Why do children have great difficulty changing the frame in which their hypotheses are constructed? Extension of the specific content of the studies from reasoning about the physical and technological would to reasoning about social domains. Is the process of hypothesis formulation and testing similar in social and non- social domains? Development of the model: We will move our model from the hand-simulation stage to the running program stage so that we can determine its internal consistency and sufficiency, and so we can explicate several of the component steps that are, at present, described imprecisely.